Re: SG Faded Pickups
Well, the Invader has a giant swath of mids, which isn't quite what I think of when I think of Megadeth or Metallica. He may have used an Invader, but to me, it seems like the inherent sound of that pickup had been mostly EQ'd out.
Megadeth is all about the mids. And Marshall 100W EL34 power sections. Give a listen to any of the early stuff with Chris Poland, "Peace Sells" in particular. Though my favorite Megadeth speed metal rhythm sound is probably when he was using VHTs. One thing I love about Megadeth is you can actually hear the bass... and with a clanky presence, not just bass doubling rhythm parts.
Metallica, the mids are chopped out post-preamp with either a Mesa 5 band EQ or, later on, a parametric EQ with narrow Q. Letting the mids distort in the preamp before notching them out distorts differently, gives a different sound than EQing them out first.
The pickups do matter less than the amp for Metallica, you definitely want a hot preamp in a Mesa Mark-series vein. Though by second record Hetfield was slaving Marshall power sections, and later he mixed various amps in (Recto, Marshall Mode 4, Bogners, Diezels, IIRC).
Assuming your amp has enough gain, and you have a good post-EQ (trying to use the mid control on the amp to scoop the mids ends up with a weak, hollow sound, yet an awful lot of imitators stop there...), and the right speakers (V30s or any midrangy, punchy Celestions), the next thing to look at probably is your pickups.
Mustaine has nearly always used JBs or a variant (active pickups modelled on JB, JB with fewer winds and/or other tweaks to coil or magnet). I'd be a bit concerned about a JB going all-honk in an SG, though. I'd want something with a bit more lower mids. Maybe an Alternative 8?
Hetfield did have an Invader in one guitar, but it's not clear if he recorded with it or how much he used it. Before the switch to EMGs, he's more associated with Gibson Dirty Fingers or 500T. Where a Dirty Fingers is somewhat like a harsher, brighter Invader, and the 500T like a grainier Distortion with a bit more low end. IIRC, he's also thought to have had DiMarzio Super Distortions in some of his guitars during RtL & MoP, at least live.
For Metallica, in an SG, you could probably use any number of Seymour Duncan pickups just fine. Any of the 44AWG ceramic magnet metal pickups. Invader might have too much bass, especially for
Master of Puppets-era. A Distortion might not have enough bass in an SG. A Black Winter might be nice if you favor tight and controllable, and a bit more flexible, but it's a bit lower output. But I'd pick a PATB-2 Parallel Axis Distortion, because it has more lower mid growl without being flubby.
PATB-2 is a crazy hot gonzo metal pickup, with crazy harmonics and lower string pull (more physical sustain) from the polepiece structure. I love it for fat riffs, chunky rhythm and blazing leads. And it's a bit less harsh on the ceramic attack, so should work reasonably for Megadeth, despite not being an A5 JB.
But unless you are chasing just metal tones, tweaking pickup height and polepiece heights, and the magnet if that isn't enough, will be more flexible. But I suspect you may prefer a hotter bridge humbucker. If you had a 498T or one of Gibson's 13Kish relatives, that'd be one thing. Those have a lot beefier lower mids, and work fine for a lot of styles, once dialed in better. Something like a Custom is pretty killer for a broad range of rock and metal (and is no slouch for speed metal rhythm and riffs, though it doesn't have the screaming upper mids thing of the ones I suggested for Metallica). IIRC, Mustaine used a Custom with the VHTs on Symphony of Destruction (but decided he preferred JBs for leads).
